As a fuel cell, there is conventionally known a solid oxide fuel cell (abbreviated as “SOFC”) having a solid electrolyte layer (solid oxide layer). The fuel cell is used in the form of a fuel cell stack having stacked therein a plurality of single cells, each of which serves as a minimum unit for power generation. The single cells are provided with air electrodes, fuel electrodes and solid electrolyte layers, respectively, and generate electric power by power generation reaction. The fuel cell stack includes, in addition to the single cells, a plurality of metal frames, insulating frames, separators and interconnectors stacked together.
The metal frames are formed of a conductive material such as stainless steel in a frame shape so as to surround peripheral surfaces of the single cells. The separators are formed of a conductive material such as stainless steel in a rectangular frame shape, with openings made in the centers thereof, and are joined to outer peripheries of the single cells so as to accommodate the single cells inside the openings of the separators. Herein, each of the separators functions as a partition plate to separate air and fuel chambers to which reaction gases (oxidant gas and fuel gas) are supplied. The interconnectors are formed of a conductive material such as stainless steel in a plate shape and are arranged on both sides of the single cells in a thickness direction so as to provide electrical conduction between the single cells.
In the case of assembling conventional flat solid oxide fuel cells (see, for example, Patent Document 1) into a fuel cell stack, it has been common practice to ensure the sealing of the fuel cell stack by a compression seal. However, the compression seal raises a concern that the utilization rate of reaction gases deteriorates due to high tendency of reaction gas leakage, excessive deformation of separators and the like.
Patent Document 2 discloses a fuel cell stack in which separators are joined by welding. It is effective to join the metal components (separators, metal frames, interconnectors etc.) of the fuel cell stack by welding into respective assembly units (called “fuel cell cassettes”) for prevention of external leakage of the reaction gases.
The fuel cell stack has manifolds defined therein through which the reaction gases flow. More specifically, the manifolds are defined in the fuel cell stack so as to extend in a stacking direction of the single cells and, at the same time, provide communication with (e.g. divide into or gather from) the respective single cells. In other words, a plurality of openings are made in the separators, the metal frames and the interconnectors such that stack through holes of the manifolds are constituted by these openings. For joining of the separator, the metal frame and the interconnector, laser welding is performed around a periphery of the metal frame and peripheries of the openings. In the resulting fuel cell cassette, the fuel chamber-side anode space to which the fuel gas is supplied is sealed by such welding.
As shown in FIG. 17, the fuel cell stack further includes insulating frames 112 each formed of a mica sheet, at a position on the separator 110 and outside the cathode space (air chamber), and having formed therein gas channels 114 for flow of the oxidant gas (air).